Weakness Revealed in the GMB and Spectra Mechanic Fuel pumps

[zKars]

I've recently been through the wringer on mechanical fuel pumps. I LOVE the stock mechanical pumps and dearly wish to be able to use them more. Silent, plenty of volume to run triples etc up to 250 hp (my educated guess). Stock new pumps put out a CRAP load of volume even at cranking speeds. 

The cheap "offshore" GMB and Spectra fuel pumps that are out there that look like the stock Nikki/Ampco/Kyosan Densi  pumps have been reported here and elsewhere are a ticket to very early failure. I can further attest to this experience. Had a fresh GMB make it about 10 km before leaving a customer/friend stranded.

So after much investigation of the issue, I have good news. 

1. The GMB and more expensive Spectra are identical inside. The diaphragm and check valves are visually and texturally identical. Might as well buy the GMB ($18.72 CAD on Rock Auto)

2. The problem is with the check valves. The design of the flapper is weak and shitty. The OEM diaphragms are thick and strong and seem fine. The failure is that they stop (or barely start ) being able to pull fuel from the tank. Any air gets in there and they loose prime. If you actuate the pump by hand on the bench in a vise with your finger over the intake, you can barely feel any suction. Do that with OEM pump and it will suck the skin off your finger tip. (Air intake only, no liquid). 

Now the really good news happens when you want to rebuild your old OEM Pump. Up till now it's been tough to find rebuild parts.

I took apart about 20 OEM pumps (all three brands, 14 Z and 6 510 ) and noticed the following.

1. Diaphragms; Depending on age and use, some were fine, some were hard and/or cracked. No surprise. About 40/60 good/bad.

2. Upper diaphragm. Not even sure what the function is of the upper rubber diaphragm, but 100% were soft and reusable. Don't think they endure much hardship. Maybe one was a little stiffer than the others. I'm sure we could fill an long thread discussing what its for.... Please don't here.

3. Check Valves. The real surprise. 100% of them were 100% perfect and appeared nearly brand new. All were clean, no sign of crud buildup or damage or erosion/corrosion. These things are indestructible apparently and totally un-affected by fuel or time.

4. Lower seals. Where the rod passes through to the actuator. About 4 out of the 20 were still soft and usable. Lots of splits and hardness.

Ok, knowing what we know about the new copies (GMB and Spectra) and the old faithfuls, and what's wrong and right about both species, is anyone seeing the possibilities here?

 

OEM Body, check valves removed

OEM check valves, front and back. Thick solid valve body. 

GMB check valves, front and back. Thin rubbery floppy material.

GMB check valves are held in place with punched nibbins. Very hard to replace the valves. Well, easy to get out, but how do you keep new ones in?

You'd have to drill and tap a pair of center holes to use the stock center hold down bracket thingy. Do-able, but....

Are these held in tightly against the gasket? Is there a gasket? Do they come loose with use and abuse? Inquiring minds need to know!

GMB mid case with their check valves in place, top view

back side of GMB upper hosing. Note different shape of center beam.

Top of GMB pump housing. Fuzzy picture like the whole situation.

 

Edited August 16, 2020 by zKars

[zKars]

What you do, is go buy the $19 GMB, replace its check valves with your old perfectly good OEM ones, and put that hybrid baby back in the car for millions of additional miles and smiles.

(Please put the GMB check valves on the cement floor and crush them into dust with a large hammer, lest you get tempted to use them at a later date. Don't)

You get a new diaphragm, new lower seal, your perfectly good check valves, and the new GMB upper diaphragm that all make up a nearly new unit, and should run for a few years pump.

Ok, all is not perfect here, there are a couple of details you need to deal with, but its all manageable. I'll explain in a but once I take some pictures. 

I guess the risk is that the GMB diaphragm material will fail faster than the OEM (or the new Nikki ones you get with Nikki pumps for non Z's to steal their internals from, remember that thread?) that is still to be seen, but all in all, maybe we have a cheaper route to rebuilt pumps here.

 

[zKars]

First thing to say about the "details" is that all the OEM pumps I took apart and scavenged parts from are identical as far as the location and size of the bolt circle that bolts the main body halves together and the top cap in place. So you can interchange just about any part with any part from any brand.  

The GMB and Spectra ALSO use the same bolt patterns. So you can interchange their bits with the OEM bits. 

There are small gotcha's here. Some of the upper caps are not interchangeable across all brands as the divider wall in the center is not in the same clocking with the bolt pattern. All you have to do is use a matching cover and middle housing from any one brand.

Next is a small annoyance that the way the check valves are held into the middle housings is VERY different with the GMB/Spectra. Cheaper and crappier of course. They use a 4 point punch around the perimiter to trap the check valves into the housing. Hard to get them out, and if you did, you couldn't get your OEM check valves to stay in as you'd have a heck of a time re-punching around the edge to keep them tight and in place. Pictures coming.

SO,. It actually makes sense to use the check valve housing and its matching cap from your old OEM pump and bolt it to the GMB/Spectra base housing. Keep all the good bits together. It does mean you have to do so sort of clean up of your OEM castings to match the new GMB castings if you care about that stuff

Edited June 5, 2020 by zKars

[zKars]

The last "gotcha" is the tiny thin seal washer between the check valve and its seat in the housing. I was able to remove virtually NONE of the OEM ones. They were hard and stiff and stuck in real well. They were easy to scrap out, but were always destroyed in the process.

So you may need to source or make some new ones. 3/4 OD (19mm) and about 18mm ID. Only about 0.05 thickness. 

A tiny ring of RTV would do as well.... You want that check valve sealed to the housings. NO LEAKS ALLOWED around the check valves.

 

Edited June 5, 2020 by zKars

[zKars]

In the 2 or 3 I've done so far, the amount of suction is spectabulus (yes I invented a word, no beer involved) so there is no problem with getting fuel from tank with an air filled line. Only time will tell about real world performance and life.

The only thing I screwed up was getting the check valves in the right way the first time. And maybe the second....

Edited June 5, 2020 by zKars

[Zed Head]

Knowledge Base material...

[jitenshakun]

Aww I'm a "/friend". 

[AK260]

Brilliant write up!!! Thank you! [emoji106][emoji106]

[Carl Beck]

On 6/4/2020 at 10:18 PM, zKars said:

I've recently been through the wringer on mechanical fuel pumps. I LOVE the stock mechanical pumps and dearly wish to be able to use them more. Silent, plenty of volume to run triples etc up to 250 hp (my educated guess). Stock new pumps put out a CRAP load of volume even at cranking speeds. 

The cheap "offshore" GMB and Spectra fuel pumps that are out there that look like the stock Nikki/Ampco/Kyosan Densi  pumps have been reported here and elsewhere are a ticket to very early failure. I can further attest to this experience. Had a fresh GMB make it about 10 km before leaving a customer/friend stranded.

So after much investigation of the issue, I have good news. 

1. The GMB and more expensive Spectra are identical inside. The diaphragm and check valves are visually and texturally identical. Might as well buy the GMB ($18.72 CAD on Rock Auto)

2. The problem is with the check valves. The design of the flapper is weak and shitty. The diaphragms are thick and strong and seem fine. The failure is that they stop (or barely start ) being able to pull fuel from the tank. Any air gets in there and they loose prime. If you actuate the pump by hand on the bench in a vise with your finger over the intake, you can barely feel any suction. Do that with OEM pump and it will suck the skin off your finger tip. (Air intake only, no liquid). 

Now the really good news happens when you want to rebuild your old OEM Pump. Up till now it's been tough to find rebuild parts.

I took apart about 20 OEM pumps (all three brands, 14 Z and 6 510 ) and noticed the following.

1. Diaphragms; Depending on age and use, some were fine, some were hard and/or cracked. No surprise. About 40/60 good/bad.

2. Upper diaphragm. Not even sure what the function is of the upper rubber diaphragm, but 100% were soft and reusable. Don't think they endure much hardship. Maybe one was a little stiffer than the others. I'm sure we could fill an long thread discussing what its for.... Please don't here.

3. Check Valves. The real surprise. 100% of them were 100% perfect and appeared nearly brand new. All were clean, no sign of crud buildup or damage or erosion/corrosion. These things are indestructible apparently and totally un-affected by fuel or time.

4. Lower seals. Where the rod passes through to the actuator. About 4 out of the 20 were still soft and usable. Lots of splits and hardness.

 

Ok, knowing what we know about the new copies (GMB and Spectra) and the old faithfuls, and what's wrong and right about both species, is anyone seeing the possibilities here?

 

 

 

 

 

 

 

 

What happened to the Pictures/Images that were Posted here?

thanks,

Carl

 

[zKars]

Not sure what is up with the pictures. I uploaded to the site, they were not on some other service or web site. 

@Mikecan you have a look and see what might be up?

[David Downs]

Today I rebuilt 3 Kyson Denki pumps using the Delphi/GMB internals. I used the original housing and swapped out all the rubber diaphragms. The check valves were removed, cleaned, and reinstalled. My o'rings were is great shape but I did install some Permatex fuel resistant gasket dressing to make sure there was a good seal. Next, I tested each pump manually by pumping fuel. Installed one and she runs great. I did notice while disassembling the Delphi/GMB pumps that the screws were a little loose which could be the reason some of them fail. I had another Delphi/GMB pump with a bad check valve. As mentioned, the Ampco and Atsugi have different internals and the top diaphragm holes do not line up. 

 

[SteveJ]

I think I'll be doing that soon for Bill's car, @David Downs. I have the carbs tuned nice, but it is starving for fuel.